"In our case we looked at groups that failed to keep up with the changes and went extinct."

Little is known about the process by which groups of animals go extinct over geological time. Marshall and colleagues set out to test a number of hypotheses on how terrestrial mammals go extinct.

One hypothesis is that species go extinct due to chance -- just like some gamblers in a casino lose. Against this 'gamblers ruin' idea is the 'Red Queen' hypothesis of evolution.

It states species must evolve as fast as they can to keep pace with a deteriorating environment.

The Red Queen refers to the character in Lewis Carroll's Through the Looking Glass, who said "It takes all the running you can do, to keep in the same place."

"What we mean by the Red Queen is the deteriorating environment," says Marshall.

Another hypothesis tested by Marshall and colleagues is that the rate of extinction of species or genera is the main driver behind the loss of diversity in mammal groups, or their ultimate demise.

Ancient camels, rhinos and elephants

Marshall and colleagues analysed the fossil records of 19 families of mammals dating back 66 million years, including those that gave rise to camels, horses, rhinos, elephants, hyenas, and some animals that have no modern descendants.

Each family began with just one genus and then its diversity waxed and waned over time, with some completely dying out all together.

Using the dates when various genera first appeared in the fossil record and when they last appeared, Marshall and colleagues found that diversity rose and fell in a way that could not be attributed to chance alone.

"The rise and fall we could show was due to more than just dumb luck," says Marshall.

The findings support the Red Queen hypothesis, although he says it's difficult to distinguish between what aspect of the environment was deteriorating and affecting diversity.

"It may have been a change in climate, vegetation, predation, parasites or competition levels," says Marshall.

The researchers also found that the rate of extinction was not the only factor that determined whether families died out or lost diversity.

"A failure to give rise to new genera was just as important as an accelerated extinction of existing ones," says Marshall.

"Even if the extinction rate stays the same but you fail to replace species, groups would disappear."

Marshall says the findings suggest that it is inappropriate to see species rising and falling as a function of whether they have reached a 'carrying capacity' that is in 'equilibrium' or 'balance' with nature.

He says this is because the 'carry capacity' for any one animal is not set but changes with the rapidly changing environment.